Method for electrochemical machining

ABSTRACT

An electrode used as a tool in an electrochemical machining process to generate raised areas or ridges in the walls of a predrilled hole in an electrically conductive workpiece has an electrically conductive cylinder partially coated with an electrically insulating material in a pattern defining the raised areas to be formed. The pattern may comprise a plurality of spaced apart rings. An electrochemical machining method of drilling bulbs in the walls of a predrilled hole uses the electrode of the invention to greatly increase process efficiency.

This application is a division of application Ser. No. 09/187,663, filedNov. 5, 1998, still pending which is hereby incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

This invention relates to a tool and a method used for electrochemicalmachining. More particularly, this invention relates to a tool andmethod for forming features in predrilled holes using electrochemicalmachining.

A specialized adaptation of electrochemical machining, known asshaped-tube electrochemical machining (STEM), is used for drillingsmall, deep holes in electrically conductive materials. STEM is anoncontact electrochemical drilling process that can produce holes withaspect ratios as high as 300:1. It is the only known method which iscapable of manufacturing the small, deep holes used for cooling bladesof efficient gas turbines.

The efficiency of a gas turbine engine is directly proportional to thetemperature of turbine gases channeled from the combustor of the engineand flowing over the turbine blades. For example, for gas turbineengines having relatively large blades, turbine gas temperaturesapproaching 2,700° F. are typical. To withstand such high temperatures,these large blades are manufactured from advanced materials andtypically include state-of-the-art type cooling features.

A turbine blade is typically cooled using a coolant such as compressordischarge air. The blade typically includes a cooling hole through whichthe air passes. A further design advancement has been the addition ofinternal ridges in the cooling hole to effect turbulent flow through thehole and increase cooling efficiency. Cooling features within the holesuch as turbulence promoting ribs, or turbulators, thus increase theefficiency of the turbine.

The cooling holes commonly have an aspect ratio, or depth to diameterratio, as large as 300:1, with a diameter as small as a few millimeters.The turbulators extend from sidewalls of the hole into the air passageabout 0.2 mm., for example.

The method currently used for drilling the cooling holes in turbineblades is a shaped-tube electrochemical machining (STEM) process. Inthis process, an electrically conductive workpiece is situated in afixed position relative to a movable manifold. The manifold supports aplurality of drilling tubes, each of which are utilized to form anaperture in the workpiece. The drilling tubes function as cathodes inthe electrochemical machining process, while the workpiece acts as theanode. As the workpiece is flooded with an electrolyte solution from thedrilling tubes, material is deplated from the workpiece in the vicinityof the leading edge of the drilling tubes to form holes.

Turbulated ridges are formed in the cooling holes by a modification ofthe standard shaped-tube electrochemical machining (STEM) process fordrilling straight-walled holes. One common method is termed cyclicdwelling. With this technique, the drilling tube is first fed forward,and then the advance is slowed or stopped in a cyclic manner. Thedwelling of the tool that occurs when the feed rate is decreased orstopped creates a local enlargement of the hole diameter, or a bulb. Thecyclic dwelling, for which cyclical voltage changes may be required,causes ridges to be formed between axially spaced bulbs. These ridgesare the turbulators.

The cyclic dwelling method is very low in process efficiency compared toshaped-tube electrochemical machining (STEM) drilling of straight-walledholes because of the lengthy required time for drilling each bulbindividually by cyclic tool dwelling. The dwell time required to form asingle bulb can be greater than the time for drilling an entirestraight-walled hole.

U.S. Pat. No. 5,306,401 describes a method for drilling cooling holes inturbine blades that uses a complex tool resetting cycle for eachturbulator in the hole. This method also has low process efficiency,having even longer operating times for drilling the turbulator ridgesthan the cyclic dwelling method because of the time required to resetthe electrode tool.

In addition, both the cyclic dwelling method and the method disclosed inU.S. Pat. No. 5,306,401 require that additional equipment be used with astandard STEM machine for control of machine ram accuracy, electrolyteflow and power supply consistency, since these are crucial to holequality. Failure to control the dimensions of the turbulated holes oftenleads to part rejection, adding significant manufacturing costs for themachining process.

Accordingly, there is a need in the art for a new and improved methodfor manufacturing turbulators that has a relatively short machiningcycle time. There is an additional need for an improved method ofmanufacturing more complex features such as spiral or helical ridges andthe like. There is an additional need for a method utilizing relativelysimple and easily implemented manufacturing techniques. In particular,there is a need for a method that does not require complex lateral orvertical displacement of the electrode.

SUMMARY OF THE INVENTION

The present invention provides an electrode for use in anelectrochemical machining process, particularly shaped-tubeelectrochemical machining (STEM), and a method of machining bulbs andridges in a predrilled hole using the electrode. The electrode andmethods of the invention provide for convenient, cost effectivemachining of features in holes with large aspect ratios. This isaccomplished by simultaneous machining of the bulbs using a shaped-tubeelectrochemical machining (STEM) process with a modified electrode.

The electrode of the present invention has an electrically conductivebody with an external surface partially coated with an insulatingmaterial in a pattern defining raised areas to be formed on the internalsurface of a predrilled hole in a workpiece. The electrode may be solidor hollow.

The electrochemical machining process of the present invention forms araised area in a surface of a predrilled hole in a workpiece. Theprocess includes the steps of positioning, in the holes, an electrodecoated with an insulating material in a pattern defining the raised areato be formed in the hole, and machining at least one bulb in theinterior surface of the hole by passing an electric current between theelectrode situated in the hole and the workpiece while circulating anelectrolyte solution through the hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conventional shaped-tubeelectrochemical machining (STEM) electrode;

FIG. 2 is a schematic representation of an electrode coated with aninsulating dielectric material in a pattern defining raised areas orridges to be machined in a predrilled straight-walled hole, inaccordance with the principles of one embodiment of the presentinvention;

FIG. 3 is a schematic representation of the hole shown in FIG. 2 afterthe raised areas have been formed with an electrode of the presentinvention;

FIG. 4 is a schematic representation of an electrode of the presentinvention that is solid, situated in a hole and includes a locator toposition the electrode in the hole;

FIG. 5 is a cross-sectional view through a locator;

FIG. 6 is a schematic representation of an electrode of the presentinvention that is hollow, and situated in a blocked hole; and

FIG. 7 is a schematic representation of another embodiment of anelectode in accordance with the instant invention.

DETAILED DESCRIPTION

A better understanding of the invention may be gained by reference tothe drawings. FIG. 1 is a schematic view illustrating a conventionalshaped-tube electrochemical machining (STEM) electrode 10 and theoperation of electrode 10 in electrochemically machining a hole 8 havinginterior walls 9 in an electrically conductive workpiece 20. Electrode10 of the prior art is a hollow metal tube 11 coated on an exteriorsurface with an insulating dielectric material 12 except at the endproximate to electrically conductive workpiece 20, where a band 14 ofexposed metal is disposed. During the drilling operation, an electrolytesolution is continuously circulated through the body of electrode 10 andhole 8 while an electrical potential is applied between electrode 10 andworkpiece 20. The solution is pumped to an inlet 16 at the end ofelectrode 10 opposite the end composed of band 14 of exposed metal,through the body of electrode 10, and through an end hole 18, which endhole 18 is enclosed by band 14 of exposed metal, through hole 8 and outof the upper end of hole 8, to be collected in a sump (not shown). Thedirection of electrolyte circulation is shown generally by arrows 13 and15.

Electric current passes between band 14 of exposed metal electrode 10and a portion of wall 9 of hole 8 directly adjacent to band 14 ofexposed metal, resulting in removal of metal from that area of wall 9.Electrical insulation by dielectric material 12 blocks the current flowin coated areas 17 on the exterior surface of electrode 10, so that nodeplating occurs in the areas of wall 9 opposite coated areas 17. Theelectrolyte solution dissolves the deplated metal and carries thedissolved metal out of hole 8. Because of the geometry of the exposedconductive surface of electrode 10, a current is established primarilyin a lateral direction toward wall 9. Current density decreases as thedistance between wall 9 and band 14 of exposed metal of electrode 10increases due to material dissolution, limiting the depth drilled. Inaddition, operating conditions such as total machining time, pulseamplitude, pulse on-time, and pulse off-time determine the totalelectrical charges passing through the machined areas, which operatingconditions in turn determine the amount of metal removal. As is known,these parameters, along with the nature and concentration of theelectrolyte and the operating voltage determine the diameter of hole 8.

The conventional method of forming raised areas such as ribs or ridgesin hole 8 is to remove metal from areas of hole 8 adjacent to thedesired location of the raised area to form bulbs 32 by a modifiedshaped-tube electrochemical machining (STEM) process. The cyclicdwelling method of the prior art uses a cyclically varying feed rate toform bulbs 32 of diameter greater than that of the straight portion 30of the hole. FIG. 1 shows the cyclic dwelling method schematically. Thefeed rate is relatively fast when drilling straight portion 30 of thehole, and relatively slow when drilling bulbs 32. Similarly, cyclicvariation of voltage can cause formation of bulbs, or enhance thebulbing process. However, cyclic variation of voltage requires asophisticated power output.

The electrode and methods of the present invention provide forconvenient, cost effective machining of features in holes with largeaspect ratios. Examples of the features that may be produced areturbulators in cooling holes in turbine airfoils, rifling in gunbarrels, and grooves in air bearing shafts.

With the improved electrode and machining process of the invention, itis possible to machine as many bulbs as desired, in whateverconfiguration desired, while achieving a significant reduction inprocess time. Furthermore, no variation of process parameters such asfeed rate or voltage are needed; therefore, costly sophisticatedcontrols for the instrument are not required.

FIG. 2 depicts an electrode 100 in accordance with one embodiment of theinvention in a predrilled hole 101 having a straight wall 102, of anelectrically conductive workpiece 110. FIG. 3 shows electrode 100 in thesame hole 101 after bulbs 120 and intervening raised areas, or ridges122, have been created. In the embodiment shown in FIGS. 2 and 3,electrode 100 comprises a hollow cylindrical electrically conductivecylinder 105 coated with an electrically insulating coating 103 in apattern having intervening areas 104 of exposed metal or conductivematerial on the exterior surface. The pattern of insulating coating 103defines raised areas or ridges to be machined in predrilled hole 101. Inthis embodiment, the pattern is a series of rings 106. The (+) and (−)designations indicate pulsed voltage through the body of electrode 100and workpiece 110.

As shown in FIG. 3, areas of exposed conductive material 104 on thesurface of electrode 100 define areas where bulbs 120 are formed byremoval of metal from wall 102 of hole 101. Raised areas or ridges 122are created in wall 102 of hole 101 where no deplating occurs in thevicinity of insulated portions 106 of the surface of electrode 100.

FIGS. 2 and 3 depict an embodiment of the invention where electrode 100consists of cylinder 105, having a body composed of an electricallyconductive material. The diameter of cylinder 105 may be as small or aslarge as necessary to fit the predrilled hole. However, the outsidediameter of cylinder 105, measured over the coated surface, typicallyranges between about 1 mm to about 8 mm. The thickness of coating 103 istypically in the range between about 0.15 to about 0.2 mm thick.

Cylinder 105 allows for pumping of an electrolyte solution into hole 101through an inlet 112 at the end of electrode 100 extending outside hole101 and out of end hole 114 at the other end of electrode 100. Inlet 112and end hole 114 facilitate uniform electrolyte flow through the areasbeing machined. Electrode 100 may also have electrolyte outlets 116along the exposed surface of electrode 100. Outlets 116 in addition toend hole 114 may be desirable where relatively large areas are beingmachined. The size of outlets 116 determines the added amount ofelectrolyte supplied to machining areas, which in turn determinessurface quality of the bulbs 120 as well as metal removal uniformity.

The operation of a shaped-tube electrochemical machining (STEM)instrument with an electrode of the present invention is similar to thatwith a conventional electrode. Current is provided by coupling electrode100 to a negative terminal of a STEM power supply (not shown) andworkpiece 110 to a positive terminal. Electrode 100 is positioned insidesmooth-walled hole 101 obtained from a previous drilling step. Anelectrolyte solution, which solution may be the same electrolyte as usedin the first drilling step, is pumped into an end of hole 101 underpressure. Where electrode 100 is hollow and may contain outlets 116 forthe electrolyte, the solution is pumped into inlet 112 of electrode 100.In this embodiment, the electrolyte flows into inlet 112 and out throughoutlets 116 along the side surface of electrode 100 and end hole 114.All raised areas or ridges as defined by the pattern of the coating ofelectrode 100 may be formed in hole 101 simultaneously.

The body of electrode 100 of the invention is composed of a conductivematerial, preferably titanium because of titanium's resistance toelectrolytic action. The outer surface of the electrode body is coveredwith an electrically insulating coating 103 in a pattern that leavessome areas of the surface exposing the conductive material of the body.Coating 103 is made of a dielectric material, which dielectric materialshould preferably be smooth, of even thickness, tightly adhered to thesurface of the body and free of pinholes or foreign material. Exemplarydielectric materials suitable for electrode 100 of the present inventioninclude polyethylene, polytetrafluoro-ethylene, ceramics, and rubbers. Apreferred method for fabricating electrode 100 of the present inventionis disclosed in a commonly assigned U.S. patent application entitled APROCESS FOR FABRICATING A TOOL USED IN ELECTROCHEMICAL MACHINING filedconcurrently herewith under Ser. No, 09/187,664, the entire contents ofwhich is incorporated by reference herein.

The pattern in coating 103 on the electrode body of the presentinvention defines raised areas or ridges 122 to be formed in predrilledhole 101. A preferred pattern is at least one ring 106 or bandcircumferentially disposed on the external surface of electrode 100. Amore preferred pattern is a series of rings or bands 106circumferentially disposed on the external surface of electrode 100. Thepresent invention, however, contemplates employing any patternconfiguration desired. Examples of other configurations that may beemployed are lines, rings or bands longitudinally disposed along theexternal surface of electrode 100. Additional configurations that may beemployed are steps or staircases, and one or more spirals or helices.The geometric components of the pattern may also be disposedorthogonally or obliquely, relative to a longitudinal axis 107 ofelectrode 100.

FIG. 4 illustrates another embodiment of the invention where anelectrode 140 is solid and may include a locator 144 at one end. Thefunction of locator 144 is to position electrode 140 in hole 101properly, such that electrode 140 is coaxial with the walls of hole 101.Locator 144 is preferably composed of the same material(s) as aninsulating coating 141 in other areas on the exterior surface ofelectrode 140, differing only in the thickness of coating 141. Theoutside diameter of electrode 140 measured at locator 144 is less thanthe inside diameter of hole 101. This outside diameter should besufficiently small that electrode 140 may be easily inserted in hole101, but sufficiently large so that electrode 140 fits snugly withinhole 101. Locator 144 preferably comprises a coating of greaterthickness compared to coating 141 on other parts of electrode 140. Forexample, the thickness of the coating 141 is typically in the rangebetween about 50 to about 75 microns, while locator 144 typicallycomprises a thickness in the range between about 100 to about 150microns.

FIG. 5 depicts a cross-section of a locator 150 in a noncircular hole151. Locator 150 should have at least three points on a surface incontact with wall 154 of hole 151, and should allow for free flow ofelectrolyte through hole 151. Exemplary locator 150 has four arms 152 incontact with wall 154 of hole 151. Electrolyte flows through spaces 156between arms 152. No metal is exposed between arms 152.

A locator is preferably disposed near the end of electrode 100 insertedin hole 101. Where the cross section of hole 101 is not circular, it maybe desirable to provide additional locator(s) 145, to aid in centeringelectrode 100 in hole 101. A preferred position for such an additionallocator 145 is at a midsection of electrode 100 as shown in FIG. 6.

The electrode and method of the invention may be used with a workpiecehaving blind (i.e. non-through) holes or through holes. As describedabove, uniform electrolyte flow is important for ensuring surface aswell as metal removal uniformity. In one embodiment of the invention,uniform electrolyte flow through a blind hole is provided for. This isillustrated in FIG. 3. The electrolyte solution is preferably passedthrough the interior of a hollow electrode 100, into hole 101 and out ofthe opening at the upper end of hole 101 and is collected in a suitablesump (not shown).

For through holes, or holes with more than one opening, some measure ispreferably taken to ensure uniform electrolyte flow inside hole 101.Through holes are commonly used in gas turbine blades. For example, thecooling holes that are frequently manufactured in such blades usingshaped-tube electrochemical machining (STEM) have an inlet and an outletfor the flow of coolant.

One method to ensure uniform electrolyte flow in a through hole is toblock one end of the hole. FIG. 6 illustrates this method, with athrough hole blocked with a plug 162 of suitable material, for example,rubber. Using this method, the electrolyte solution may be passedthrough a hollow electrode 100 such as that depicted in FIGS. 2 and 3.The outlet(s) for the solution may be located either along the side orat the lower end of electrode 100. Where the electrode is solid and thepredrilled hole is a through hole, electrolyte solution may be pumped inone end of the hole and out the other end.

FIG. 4 shows the second method to ensure uniform electrolyte flow in athrough hole where the electrode is solid. Electrode 140 consists of asolid body 145 coated with a suitable dielectric material 141 in apattern, leaving areas where electrically conductive material of thebody is exposed, and a locator 144. Using this method, electrolyte ispumped, for example, from the lower end of hole 101, around electrode140, and out of the upper end of the hole 101.

EXAMPLE

A straight-walled hole was drilled in a workpiece made up RD 26,175/USAof two pieces of stainless steel clamped together. The hole was drilledat the interface where the two pieces were joined using a standard STEMapparatus and a conventional electrode similar to that shown in FIG. 1.After the straight drilling was completed, an electrode according to thepresent invention, such as that illustrated in FIGS. 2 and 3, wasconnected to the STEM apparatus, and placed within the predrilled hole.A set of bulbs was simultaneously electrochemically machined in thehole, leaving raised areas, or ridges, between the bulbs. The spacing ofthe rings of insulating material in the pattern on the electrodecorrelated with the spacing of the ridges in the hole, and the width ofthe rings correlated with the width of the ridges.

While only certain features of the invention have been illustrated anddescribed, many modifications and changes will occur to those skilled inthe art. It is, therefore, to be understood that the appended claims areintended to cover all such modifications and changes as fall within thetrue spirit of the invention.

What is claimed is:
 1. An electrochemical machining process for forminga raised area in a wall of a predrilled hole in a workpiece comprising:positioning in the hole an electrode coated with an insulating materialin a pattern defining the raised area to be formed in the wall of thehole, and machining at least one bulb in the wall of the hole by passingan electric current between the electrode positioned in the hole and theworkpiece while circulating an electrolyte solution through the hole. 2.A process according to claim 1 further comprising simultaneously forminga plurality of raised areas in the surface of the hole with theelectrode.
 3. A process according to claim 2 wherein said electrode isstationarily positioned in said hole while said plurality of raisedareas are simultaneously formed with the electrode.
 4. A processaccording to claim 1 wherein the hole has a non-circular cross sectionand further comprising positioning the electrode in a center of saidhole.
 5. A process according to claim 1 farther comprising positioningthe electrode in a center of said hole with a locator associated withsaid electrode.
 6. A process according to claim 1 wherein theelectrolyte solution is passed through the electrode into the hole.
 7. Aprocess according to claim 1 wherein the electrolyte solution is passedthrough the hole around the electrode.
 8. A process according to claim 1wherein the workpiece comprises a turbine blade and the raised areacomprises a turbulator ridge.
 9. A turbine blade manufactured accordingto claim 8.